The detection of the three-dimensional spatial shape of bodies or body parts, in particular of human body parts, such as legs, torso or feet, is an important aspect in the production or assignment of fitting articles of clothing, orthopedic aids such as compression stockings, prostheses and ortheses and also in the production or assignment of fitting shoes. In addition, such body scanners are also made use of more and more frequently in the form of whole-body scanners in the field of medical diagnosis of orthopedic results such as scoliosis of the back, of therapy monitoring of lymphedemas, and of cosmetic surgical procedures such as liposuction etc.
All of these applications require “real” 3D scanners, i.e. scanners which are capable of detecting a complete 3D model both of the convex and the concave parts of the body. Therefore, in considering the prior art, pseudo 3D methods such as the silhouette method (taking images of the silhouette of a body from several directions) are excluded since these methods are not normally able to provide a complete 3D model.
The users of scanners rightly expect, for cost reasons, that the same body scanner can be utilized as far as possible for different areas of application and different tasks and that the 3D data required for a particular application is taken as a subset from the complete 3D data set prepared of the entire body. It is only in this way that an uneconomic split-up of the tasks set to a large number of specific scanners such as foot scanners, back scanners, torso scanners, knee scanners, shoulder scanners etc. can be avoided.
This demand for a whole-body scanner places high requirements on the measuring room (area of approx. 1 m2×height of 2 m) to be covered by these scanners, the necessary high resolution of typically 1000 spatial points per cubic centimeter, and the high precision of the 3D model prepared of approx. 0.5 mm, a precision that is, above all, constant in the long term.
In addition to these rather technical specifications, these whole-body scanners are required to satisfy a number of further conditions in order to be economically successfully employed in those areas that are very critical in terms of cost, i.e. in the medical field, the paramedical and sanitary medical industries, and in mass customization of clothing and protective clothing:
(a) the body scanner needs to manage with as small a floor space as possible because the space available is often very limited;
(b) the body scanner needs to be able to operate under normal ambient light conditions, i.e. without being installed in a dark and separated booth, because this leads to anxieties and reluctance in many customers;
(c) the body scanner needs to be able to be reliably operated even by staff who have received only little training. This makes it absolutely necessary to avoid any frequent and complicated recalibration processes, which would ask too much of such staff;
(d) in spite of these high requirements, manufacturing costs and operating costs must be very low to allow an application in the above-mentioned cost-sensitive fields of business.
Meanwhile, numerous optical 3D body scanners are on the market, most of which operate either on the basis of the methods of laser light section (see, e.g., Vitronic Dr. Stein, www.http://www.vitronic.de/bodyscannen) or stripe projection (see, e.g., bodyScan of Breuckmann, www.breuckmann.com). Both methods are based on triangulation, i.e. a very stable and precise spatial triangular arrangement of a light projector, a camera and a body for point-by-point determination of the distance of the body surface observed from the camera/projector measuring head. An XYZ point model of the body surface viewed is prepared from the sum of this distance data in a world-related and/or object-related coordinate system. In further processing steps, a 3D model suitable for the further processing is calculated therefrom, in most cases by a triangulated mesh, in any one of the numerous standardized formats such as .dxf, VRML, STEP etc.
In order to detect the entire body, either a plurality of camera/projector arrangements need to be mounted around the body (e.g., in the case of the bodyScan of Breuckmann) or a plurality of camera/projector arrangements need to be mechanically moved over the body surface (e.g., in the case of the VITUS whole-body scanner of Vitronic Dr. Stein).
A detailed presentation of the currently commercially available body scanners and the technologies made use of can be found via the Internet portal www.hometrica.ch.
The required angular arrangement of the camera relative to the projector is sensitive: even small angle errors result in large errors of measurement in the distances measured. The movement of the camera/projector arrangement in the space is equally sensitive: small errors in the position determination of the measuring head while an image is taken result in large errors of measurement in the 3D model generated. This sensitivity results in that, even in the case of a very sturdy and expensive opto-mechanical construction, all scanners using the principle of triangulation still require frequent recalibration, in particular also after each transportation and upon each movement of the scanner.
The necessary recalibration of a 3D scanner operating on the basis of triangulation using laser or stripe projection requires various standards such as standard bodies, marked plates etc., and generates a large number of parameters which are required in order to ensure the measuring accuracy. These include:
the exact spatial position between the camera and the projector (triangulation angle, base line, mutual orientation, etc.);
the exact internal parameters of the camera and the projector (focal lengths, sensor dimension, geometry of the picture elements, tilt angle and angle of rotation of the laser line projector, etc.);
the exact spatial positions of the camera/projector measuring head for each measuring image taken, the so-called external parameters; etc.
Therefore, as a rule, recalibration of a 3D scanner is a complicated process which in many cases is asking too much of the sales staff of, e.g., an orthopedic specialist store, who therefore tend not to accept this technology.
Because of the mechanical stability required, today's 3D whole-body scanners based on triangulation cannot be offered at a particularly low price, either, so that currently many potential applications are not commercially put to practice due to the high costs of the 3D scanners.
The company of corpus.e AG (www.corpus-e.com) has developed a photogrammetric foot scanner under the name “Lightbeam®”, which operates without a projector and without a sensitive triangulation arrangement (see www.corpus-e.com). The foot is covered with a specially photogrammetrically marked, elastic sock here and a video camera is mechanically moved around the foot (see WO 2004/078040 A1).
The foot is placed on a photogrammetrically marked support, so that the spatial position from which the camera measures can be permanently and automatically determined using the methods of photogrammetry (the so-called “external” parameters of a photogrammetric measuring arrangement). Likewise, the so-called “internal” parameters of the camera itself, such as focal length, image sensor, piercing point of the optical axis, lens distortions, etc. can be determined automatically from the evaluation of overlapping 2D images taken of the marked support and the marked foot. This makes this system completely calibration-free or, more precisely, inherently self-calibrating. It may be put into operation after transportation at any time without calibration by the user; there is no need to ever recalibrate it after a change of load; the design may be simple and inexpensive in terms of mechanical stability since the latter does not contribute to the final result, the 3D model measured.
However, this otherwise powerful method has a drawback: due to the density, which is limited owing to the textile nature, of the photogrammetric markings on the elastic sock, the density of the XYZ point cloud generated is distinctly lower in comparison with a laser or stripe projection method (typically 4000 XYZ points as against approx. 1 million XYZ points in the case of a light section scanner). While this lower point density does not constitute a disadvantage in the case of flat body parts such as the upper foot, it may be restricting in regions of high spatial curvatures such as in the region of the toes, the heel, the transition from the upper foot to the sole, etc.
DE 101 56 908 A1 also discloses a method of detecting the three-dimensional shape of a body, in which the body to be measured is completely covered with a photogrammetrically marked, elastic covering. The body is placed on a plate which includes photogrammetrically evaluatable marks. Overlapping images are taken freehand by an operator.
The requirement that the body to be measured needs to be completely covered with a photogrammetrically marked, elastic covering constitutes a further drawback. Such coverings are not simple to produce; depending on the physique of the customer and depending on the body part in question, such as the torso, legs, feet, shoulders, hands, etc., several shapes and sizes are necessary.
There are also scanning applications such as, e.g., the digitization of feet for the selection of suitable ski boots, in which it is important that the customer keeps on his/her own winter sock, for the sock to be taken into consideration in the shape adaptation. But it is not possible to photogrammetrically mark any random sock later using simple means.
Therefore, despite the freedom from calibration achieved, this system is not particularly suitable for a low-cost whole-body scanner.
DE 196536294 A1 to Malz et al. describes a general method of 3D digitization of objects with the aid of a camera/light projector triangulation arrangement in which the entire measuring room is provided with photogrammetrically marked side walls and, in addition, further photogrammetrically marked calibration bodies are placed in the measuring room. This method allows the explicit in situ calibration of the measuring arrangement.
This methodology is applicable in a measuring laboratory where well-trained staff digitize primarily rigid mechanical models such as, e.g., in automotive engineering. It is, however, not very suitable for a low-cost whole-body scanner in which the number of photogrammetrically marked surfaces in a measuring booth of a medical supply store is intended to be small and, if possible, also aesthetically non-intrusive, where no calibration bodies or panels to be manually installed should be required at all and, altogether, the impression of a closed measuring room should be avoided. This method described by Malz is also not very suitable for a whole-body scanner which is to be used for digitizing the human body all around, i.e. completely from a large number of views and as quickly as possible.
In WO 2009/065418 A1 of 28 May 2009, Rutschmann and Pfeiffer present a body scanner which likewise manages without any photogrammetrically marked textile coverings while still maintaining the desired principle of freedom from calibration (or of implicit recalibration). This method uses a photogrammetrically marked base plate on which a customer stands, and a laser line/camera triangulation arrangement mechanically moved around the body approximately on a circular path. Here the camera is oriented in such a manner that for each position on the orbit it also covers a detail of the floor markings and, at a higher position roughly in the center of the image, likewise also covers an elastic, photogrammetrically marked band which is wrapped around the customer's leg, for example. With each exposure, the camera thus simultaneously detects the following elements:
the (at least one) bright light line of the laser projector on the body surface, which runs vertically through the image field;
the same line in the lower part of the image field, which generates a bright trace on the photogrammetric base plate;
the bright trace of the light line on the elastic band roughly in the center of the image.
Provided that the single design requirement is met that the laser line generated is straight, both the internal parameters of the camera and the external parameters of the line projector/camera measuring head (triangulation angle, base line, orientation in space) can be determined from the numerous images taken by the camera around the body using known photogrammetric methods. This makes this system inherently self-calibrating; any mechanical change to the critical components, that is, the line projector and the camera, i.e. all of the internal and external parameters are photogrammetrically determined at the same time with each digitization.
This results in a significant advantage to the user: these scanners need not be recalibrated by the user. To the manufacturer this inherent self-calibration means that no extremely rigid and precise designs, no complicated adjustments etc. are necessary, making such body scanners cost-efficient to produce and simple to put into operation at the customer's.
But even with all the advantages, the method described by Rutschmann et al. still has a number of drawbacks: Due to the movement of the camera/line projector arrangement around the body of the customer, a relatively large cylindrical space is required; such a scanner typically requires a circular area of about 3 m in diameter since minimum distances between the camera/laser measuring head and the body of the customer need to be observed. Medical supply stores and hospitals often do not dispose of spaces of such size, but expect a whole-body scanner which requires as little floor space as possible. The manual application of a photogrammetrically marked band or similar mark in the image field is a source of error and requires a certain degree of optical understanding; the sales staff often lack such understanding. In addition, WO 2009/065418 A1 does not solve the problem of realizing a whole-body scanner which reaches at least from the feet up to the shoulder/neck area. Even with extreme wide angle cameras, the camera/line projector arrangement shown is not able to illuminate the top surface of the shoulder with the line projector and to capture it optically simultaneously with the photogrammetrically marked floor plate, or to capture the occluded parts of the lower belly in an image.
WO 2009/065418 A1 does also not teach how to clearly recognize the light trace of the line projector among the multiplicity of the photogrammetric marks and how to evaluate it based on the image of the camera. Deviations from the design-related rectilinearity of the line projector are not recognized and cannot be corrected automatically.
There is therefore an economic and technical interest in a cost-efficient, calibration-free or inherently recalibrated body scanner that is simple to operate, for high-resolution detection of the anatomical three-dimensional shape mainly of human bodies, which does not require a photogrammetrically marked measuring textile, which manages with a small-volume measuring setup, and automatically generates a 3D model of such precision and density that it can be utilized for a multitude of applications ranging from medical and orthopedic fields to mass customization of footwear and clothing.